Fluids in hydrocarbon reservoirs may be accessed via wellbores that extend down into the ground toward the targeted reservoirs. In some cases, the hydrocarbon reservoirs may contain fluid with a lower viscosity such that crude oil flows relatively freely from the reservoir, through production tubing, and toward the production equipment at the ground surface. Some hydrocarbon reservoirs comprise fluids having a higher viscosity, which may not freely flow from the reservoir and through the production tubing. These high viscosity fluids in the hydrocarbon reservoirs are occasionally referred to as “heavy oil deposits.” In the past, the high viscosity fluids in the hydrocarbon reservoirs remained untapped due to the inability and expense of recovering them. More recently, as the demand for crude oil has increased, commercial operations have expanded to the recovery of such heavy oil deposits.
In some circumstances, the application of heat (e.g., using steam) to the hydrocarbon reservoir may reduce the viscosity of the fluids in the reservoir so as to permit the extraction of crude oil and other liquids from the reservoir. This process is referred to as thermal recovery and can include continuous steam flooding, cyclic steam stimulation, and in-situ combustion. In steam flooding, steam is pumped into injection wells to heat up the reservoir and the heavy oil deposits contained in the reservoir. As a result, heavy oil and other liquids can be recovered through production wells.
Cyclic steam stimulation, or “huff'n puff”, involves injecting steam into a well during an injection (or huff) phase that may last days or weeks followed by a soak phase during which the well is shut in for a period of days or weeks to allow the heat to dissipate and to reduce the viscosity of the heavy oil in the reservoir. Finally, during a production (or puff) phase heavy oil is pumped from the same well used for injecting steam.
In-situ combustion involves burning subsurface oil in a reservoir. High temperature air is injected into the reservoir to initiate burning of the subsurface oil, which, in turn, heats up the reservoir and the heavy oil deposits that it contains. Water can also be injected in addition to air. The burning oil heats the water to produce steam, further heating the reservoir, reducing the viscosity of the heavy oil deposits, and allowing heavy oil to be recovered through a production well.